You can also obsess over the possibility of any tank breaking when it is in the living room on a hardwood floor. In reality that tank is either designed with sufficient safety factor while rimless, so the rim is not a structural part, or it isn't. We should be able to make that determination independently. Right? Speak up!!

I think they use the plastic rims around the tank to actually fabricate the tank faster.
Some silicone on the bottom rim, place the bottom plate on it, some silicone on the top rim, place left and right side in, and then the front and back. Close the seams with silicone and done! They probably don't need clamps to put the tank together. As it said on the sticker on the tank "made by (person)" I don't think I can put a 75 gal together by myself without wasting hours of time. Those tanks have to been made fast due to make fast money...